1. Field of the Invention
The present invention relates to a device that detects reduction in the rotational speed of a cooling fan of a machine tool.
2. Description of the Related Art
A fan is widely used as a unit to cool a heat generating portion, such as a motor and a control panel, during an operation of a machine tool. Overheating causes malfunction of a machine tool. Thus, a fan is desired to keep cooling without stopping to reduce downtime and for preventive maintenance.
Stopping modes of a fan are roughly categorized into part malfunction, such as a break of a fan motor coil and an earth fault, and mechanical restriction, such as tangling of a foreign object on a fan blade. Binding of a fan by sludge, which is a mode of the mechanical restriction, happens most frequently. When a rotating fan suctions the atmosphere including oil mist and powder dust surrounding a machine tool, oil mist and powder dust adhere on the fan in a form of sludge and hinder the rotation of the fan. As the sludge gradually grows, the fan reduces its rotational speed and eventually stops by binding.
Oil mist and powder dust contamination is also the most frequent cause of the stopping mode of the part malfunction. In many cases, the sludge is likely to cause reduction in the rotational speed before malfunction occurs. For a fan motor that is a three-phase motor, an earth fault or a break of wire happening in only one phase causes reduction in the rotational speed but the fan keeps on rotating. As contamination grows, malfunction occurs at a plurality of sections, and eventually the fan stops.
As described above, reduction in the rotational speed occurs before the fan stops. By detecting in advance the reduction in the rotational speed, stop of the fan can be prevented.
To detect the reduction in the rotational speed of the fan, an encoder that detects the rotational speed may be attached to the fan. However, an encoder is expensive compared to a fan. Moreover, the encoder is required to be attached near the fan where oil mist and powder dust suctioned by the fan accumulate, so the encoder is likely to go wrong.
Regarding this problem, Japanese Patent Application Laid-Open No. 03-169209 discloses a method for detecting the rotational speed of a fan of a controller including a fan motor by monitoring a consumption current. With this technique, the reduction in the rotational speed is detected by monitoring the increase in the consumption current as sludge hinders the rotation of the fan to require a larger torque to rotate the fan. A current measuring device is inexpensive and need not be attached near the fan. Thus, the problem related to the detection of the reduction in the rotational speed by an encoder can be solved. Furthermore, an earth fault and a break of wire can also be detected by monitoring the current.
However, the consumption current of the fan fluctuates with the change in a power source voltage. The fluctuation is not likely to occur when power is supplied to the fan from a power source circuit inside a machine tool as in the technique disclosed in Japanese Patent Application Laid-Open No. 03-169209. However, when power is directly supplied to the fan from a commercial power source, of which voltage tends to fluctuate, whether the change in the consumption current is caused by the reduction in the rotational speed or the fluctuation in the power source voltage cannot be distinguished.